In a known pressure diecasting method Bulgarian Pat. No. 187/98, particularly for casting under counter-pressure under the action of the pressure differential between the sealed chamber with the melt reservoir and the sealed chamber with the mold, the melt passes through a conduit and fills the casting mold, while during mold filling a gas counter-pressure is acting inside the sealed chamber containing the mold.
A drawback of this method is that, when the casting mold is opened, the feed conduit is connected directly to the ambient atmosphere and the melt within it is under the action of the gas contained inside the mold, or under the action of air. As a result, the castings have a number of defects which are due to the following causes: the interaction of the gas or air and the cast material; the dissolution in the melt or non-release from it of additional quantities of gas; and variations in the gas content of the produced castings.
Another drawback is that a large quantity of gas is used for effecting the operation of casting. This is not favourable from an energy viewpoint and causes, moreover, a varying quality of the subsequently produced castings. This is a result of the disturbed equilibrium between the dissolved gases and other volatile components of the melt and the partial pressures of these components in the gaseous phase over the melt.
A known low-pressure diecasting machine (French Pat. No. 2,147,827) comprises a sealed chamber inside which there is placed a crucible with molten metal. This sealed chamber is connected to a pressure chamber. The casting mold is placed over the pressure chamber. The sealed chamber and the pressure chamber are connected by a metal conduit, one end of which is immersed in the crucible with molten metal. Two conical cavities are shaped in the pressure chamber, which are interconnected and connected to the metal conduit. One conical cavity is connected in its upper end to the casting mold, while the other cavity is connected to a pipe conduit with a valve which has at least four ports. The first port of the valve is connected to a source of pressurized gas, the second--to the gas space of the sealed chamber, the third--to the pipe conduit of the second conical cavity of the pressure chamber, and the fourth--to the atmosphere.
Both conical cavities of the pressure chamber are of practically equal volume and are connected by a hole with a cross-sectional area equal to that of the metal conduit. The volume of the conical cavity which is connected to the valve is such, that when the casting mold is full, the level of the melt within this cavity remains below a preset level.
The pressure chamber is provided in its upper part, i.e. over the commencement of the pipe conduit connecting the one conical cavity to the valve, with a device which stops the gas delivery in this cavity when the melt inside it reaches the preset level.
A drawback of this machine lies in that it allows the casting of parts only under low pressure and is specially adapted to the casting of thin-walled hollow parts; this requires an additional intermediate pressure chamber with a valve for the control of the steps of the casting process.
Another drawback is that the seals between the surfaces of the different components of the machine are not protected from possible contact with the melt; this makes necessary the use of plastically deformable components which must be frequently replaced as a result of quick wear.
Another drawback of this machine is that its productivity is low because the mold is removed after the solidification of the melt and this requires considerably more time than the duration of mold filling.
Another machine (Bulgarian Pat. No. 16,793) for casting metals under gas counter-pressure comprises a sealed melt reservoir, closed by an intermediate plate which carries the feed tube and the casting die. The material feed tube is sealed to the intermediate cover by means of a plastically deformable seal, and the die is sealed to the flange of the material feed tube against leakage of melt by means of mating surfaces and a thin plastic gasket.
A drawback of this machine is its low productivity because of the several times longer time for solidification of the melt and for cooling the cast body down to the temperature of removal, as compared to the time necessary for filling the casting die. This drawback is particularly noticeable when using built-up dies in which the complex inner and outer surfaces of the casting are shaped by cores arranged in a metal box. This is also valid for the casting in combined sand-metal molds and in sand molds.
A substantial drawback of this machine is that very complex devices are required for maintaining the filling of the die at one and the same pressure at a gradual drop of the level of the melt inside the sealed reservoir. Therefore, the production of subsequent castings of constant quality is not ensured.
Another drawback is that the plastically deformable seal is not adapted for sealing against flow-out of melt during the change of the casting dies. It is not possible to ensure a sealing of the die against the flange of the material feed conduit only by pressing mating surfaces.